Study of the Spatial Response of Reduced Pitch Hg 1ÿx Cd x Te Dual-Band Detector Arrays O. GRAVRAND, 1,3 J.C. DESPLANCHES, 1 C. DELBE ` GUE, 1 G. MATHIEU, 2 and J. ROTHMAN 1 1.—CEA LETI LIR, Grenoble, 38054 France. 2.—Photoptim, Montpellier, 34080 France. 3.—E-mail: olivier.gravrand@cea.fr The third generation of infrared (IR) focal plane arrays (FPA) will have to detect simultaneously at least two different spectral bands. Small pixel pitches (less than 25–30 mm) will also be needed to achieve the resolution required for such devices without degrading performances, introducing difficult technologi- cal challenges. Among those is optical filling: the third generation will have to retain the high filling factor obtained with a planar single-color FPA. It thus appears necessary to estimate the sensible areas of small pitched dual-band pixels. Scanning a controlled IR spot on the FPA surface allows the observa- tion of the spatial response of a pixel. Such spotscan measurements have been carried out on Hg 1ÿx Cd x Te dual-band arrays. Different device structures have been tested: planar structures and NPN mesa structures, produced at LETI LIR. The spotscan bench test is first presented, in order to clarify characteri- zation conditions and understanding. Measurement results are then discussed and compared with modeling, in order to identify the main physical parameters determining the optical area of such dual-band IR detectors. Emphasis on mod- ulation transfer function (MTF) estimation and X talk is finally carried out. Key words: Spotscan, optical area, modulation transfer function (MTF), single color IR detectors, dual band IR detectors INTRODUCTION The second generation of infrared focal plane arrays (IR FPA) concerns 1d or 2d arrays of single- color detectors. This type of FPA can be realized with Hg 1ÿx Cd x Te (MCT) using an ion implantation technology to form a 2d planar array of detectors. 1 Such technology has demonstrated high perform- ance level detectors: high quantum efficiency back- ground limited infrared photodetectors. 2 The third generation is then supposed to detect simultaneously in at least two spectral bands with a small pitch 2d array configuration, keeping the high level of performance obtained with the second generation. In order to address this goal, LETI LIR has designed a MCT-based dual-band FPA mesa technology based on a four-layer heterostructure molecular beam epitaxy (MBE) grown. It has been previously shown that this type of structure leads to the same level of performances as the previous generation in terms of noise and quantum efficiency in the 3–5 mm range. 3 Because the 2d array single-color technology is a planar technology, it ensures very high optical fill factors (ratio of the effective sensitive area to squared pitch). As will be shown later in this paper, each junction is surrounded by at least four other junctions, limiting the lateral diffusion of generated photo carriers. Thus, the effective sensitive area obtained is very close to the optimum. Moreover, the presence of operating nearby junctions ensures that this optical area will not exceed the pixel area. In other words, the planar junction structure limits intrinsically spatial cross-talk. This effect will be called here ‘‘self-confinement’’ and will be explained more in detail later in this paper. The situation is slightly different in the case of dual-band 2d arrays. It is necessary to etch deep trenches in the sensitive material in order to isolate the lower band homojunction located at the base of the layered material structure. 3 On the one hand, because those trench widths are for the moment on (Received October 17, 2005; February 1, 2006) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 6, 2006 Special Issue Paper 1159